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The explanation for exact spectral lines for each substance was provided by the  quantum theory . In his 1913 model of the hydrogen atom Niels Bohr showed that the observed series of lines could be explained by assuming that electrons are restricted to atomic orbits in which their orbital angular momentum is an integral multiple of the quantity   h /2π, where  h  is Planck's constant. The integer multiple (e.g., 1, 2, 3 …) of  h /2π is usually called the quantum number and represented by the symbol  n. When an electron changes from an orbit of higher energy (higher angular momentum) to one of lower energy, a  photon  of light energy is emitted whose frequency ν is related to the energy difference Δ E  by the equation ν=Δ E / h. For hydrogen, the frequencies of the spectral lines are given by ν= cR  (1/ n f 2 −1/ n i 2 ) where  c  is the speed of light,  R is the Rydberg constant, and  n f  and  n i  are the final and initial quantum numbers of the electron orbits ( n i  is always

Spectrum, arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph ). Physicists often find it useful to separate a beam of particles into a spectrum according to their energy. Continuous and Line Spectra Dispersion, the separation of visible light into a spectrum, may be accomplished by means of a prism or a diffraction grating. Each different wavelength or frequency of visible light corresponds to a different color , so that the spectrum appears as a band of colors ranging from violet at the short-wavelength (high-frequency) end of the spectrum through indigo, blue, green, yellow, and orange, to red at the long-wavelength (low-frequency) end of the spectrum. In addition to visible light, other types of electromagnetic radiation may be spread into a spectrum according to

Conservation laws, in physics, basic laws that together determine which processes can or cannot occur in nature; each law maintains that the total value of the quantity governed by that law, e.g., mass or energy, remains unchanged during physical processes. Conservation laws have the broadest possible application of all laws in physics and are thus considered by many scientists to be the most fundamental laws in nature. Conservation of Classical Processes Most conservation laws are exact, or absolute, i.e., they apply to all possible processes; a few conservation laws are only partial, holding for some types of processes but not for others. By the beginning of the 20th cent. physics had established conservation laws governing the following quantities: energy, mass (or matter), linear momentum, angular momentum , and electric charge. When the theory of relativity showed (1905) that mass was a form of energy, the two laws governing these quantities were combined into a single l

Momentum, in mechanics, the quantity of motion of a body, specifically the product of the mass of the body and its velocity . Momentum is a vector quantity; i.e., it has both a magnitude and a direction, the direction being the same as that of the velocity vector. When an external force acts upon a body or a system of bodies in motion, it causes a change in the momentum of the body. The impulse of a force acting on a body is the product of the force and the duration of time in which it acts and is equal to the change in momentum of the body. When no external force acts upon a body in motion or a system of bodies there is no change in the total momentum even though, as in the case of a system of bodies, there may be an internal disturbance of the system resulting in changes in the momenta of individual bodies. This conclusion is commonly known as the principle of the conservation of momentum (see conservation laws , in physics). The momentum of a body should not be confused with its ki

Each element is assigned an official symbol by the International Union of Pure and Applied Chemistry (IUPAC). For example, the symbol for carbon is C, and the symbol for silver is Ag [Lat.  argentum  = silver]. There are several ways of designating an isotope. One designation consists of the name or symbol of the element followed by a hyphen and the mass number of the isotope; thus the isotope of carbon with mass number 12 can be designated carbon-12 or C-12. The mass number is often written as a superscript, e.g., C 12 ; sometimes the atomic number is written as a subscript preceding the symbol, e.g.,  6 C 12 . The IUPAC rules for nomenclature of inorganic chemistry state that the subscript atomic number and superscript mass number should both precede the symbol, e.g.,  12 6 C. Many isotopes were given special names and symbols when they were first discovered in natural radioactive decay series (e.g., uranium-235 was called actinouranium and represented by the symbol AcU). This prac

Nonmetal, chemical element possessing certain properties by which it is distinguished from a metal . In general, this distinction is drawn on the basis that a nonmetal tends to accept electrons and form negative ions and that its oxide is acidic. Nonmetals are poor conductors of heat and electricity and do not have the luster of metals. Arsenic, antimony, selenium, and tellurium exhibit both nonmetallic and metallic properties and are called metalloids. Unlike the metals, which are all solids (with the exception of mercury) under ordinary conditions of temperature and pressure, the nonmetals appear in all three states. Argon, chlorine, fluorine, helium, hydrogen, krypton, neon, nitrogen, oxygen, and xenon are normally gases. Bromine is a liquid. Boron, carbon, iodine, phosphorus, silicon, and sulfur are solids. Certain of them, e.g., boron, carbon, iodine, silicon, and sulfur, form crystals, as do the metals. In hardness they vary considerably. Carbon in its allotropic form, the dia

Properties of an element are sometimes classed as either chemical or physical. Chemical properties are usually observed in the course of a chemical reaction, while physical properties are observed by examining a sample of the pure element. The chemical properties of an element are due to the distribution of electrons around the atom's nucleus, particularly the outer, or valence, electrons; it is these electrons that are involved in chemical reactions. A chemical reaction does not affect the atomic nucleus; the atomic number therefore remains unchanged in a chemical reaction. Some properties of an element can be observed only in a collection of atoms or molecules of the element. These properties include color, density, melting point, boiling point, and thermal and electrical conductivity. While some of these properties are due chiefly to the electronic structure of the element, others are more closely related to properties of the nucleus, e.g., mass number. The elements are someti

Compound,  in chemistry, a substance composed of  atoms  of two or more  elements  in chemical combination, occurring in a fixed, definite proportion and arranged in a fixed, definite structure. A compound is often represented by its chemical formula. The formula for water is H 2 O, and for sodium chloride, NaCl. The formula weight of a compound can be determined from its formula. The molecular weight of a molecular compound can be determined from its molecular formula. Two or more distinct compounds that have the same molecular formula but different properties are called isomers. Formation and Decomposition of Compounds Compounds are formed from simpler substances by  chemical reaction . Some compounds can be formed directly from their constituent elements, e.g., water from hydrogen and oxygen: 2H 2  + O 2  → 2H 2 O. Other compounds are formed by reaction of an element with another compound; e.g., sodium hydroxide (NaOH) is formed (and hydrogen gas released) by the reaction

Synthetic elements,  in chemistry, radioactive elements that were not discovered occurring in nature but as artificially produced isotopes. They are  technetium  (at. no. 43), which was the first element to be synthesized,  promethium  (at. no. 61),  astatine  (at. no. 85),  francium  (at. no. 87), and the  transuranium elements  (at. no. 93 and beyond in the  periodic table ). Some of these elements have since been shown to exist in minute amounts in nature, usually as short-lived members of natural radioactive decay series (see  radioactivity ). The synthetic elements through at. no. 100 ( fermium ) are created by bombarding a heavy element, such as uranium or plutonium, with neutrons or alpha particles. The synthesis of the transfermium elements (elements with at. no. 101 or greater) is accomplished by the fusion of the nuclei of two lighter elements. Elements 101 through 106 were first produced by fusing the nuclei of slightly lighter elements, such as  californium , with those

Ununhexium, artificially produced radioactive chemical element ; symbol Uuh; at. no. 116; mass number of most stable isotope 292; m.p., b.p., sp. gr., and valence unknown. Situated in Group 16 of the periodic table, it is expected to have properties similar to those of polonium and tellurium . In 1999 a research team at the Lawrence Berkeley National Laboratory in Calif. bombarded lead-208 atoms with high-energy krypton-86 ions to create, apparently, ununoctium (element 118) atoms. The Uuo-293 isotope that they synthesized emitted an alpha particle to decay into Uuh-289, which has a life-life of about 0.6 millisecond, which then emitted an alpha particle to decay into ununquadium (element 114). Although the Berkeley laboratory retracted its claim for creating ununoctium in 2001, other research teams have since created ununhexium directly. No name has yet been adopted for element 116, which is therefore called ununhexium, from the Latin roots un for one and hex for six, under a con

Ununoctium (y'nənŏk`tēəm), artificially produced radioactive chemical element ; symbol Uuo; at. no. 118. Scientists from the Joint Institute for Nuclear Research in Dubna, Russia, and Lawrence Livermore National Laboratory in California collaborated in the discovery of ununoctium in experiments conducted in 2002 and 2005. They bombarded atoms of californium -249  with ions of calcium -48. Among the products of the bombardments were three atoms of ununoctium-294 (one atom in 2002 and two in 2005), each of which decayed in 0.9 milliseconds into an atom of ununhexium by emitting an alpha particle . No name has yet been adopted for element 118, which is therefore called ununoctium, from the Latin roots un for one and oct for eight, under a convention for neutral temporary names proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1980. In 1999 a research team at the Lawrence Berkeley National Laboratory in Calif. bombarded lead-208 atoms with high-energy k

Element Symbol Atomic Number Atomic Weight 1 Melting Point (Degrees Celsius) Boiling Point (Degrees Celsius) actinium Ac 89 227.0278 1050. 3200. ±300 aluminum Al 13 26.98154 660.37 2467. americium Am 95 (243) 1172. 2600. antimony Sb 51 121.75 630.74 1750. argon Ar 18 39.948 −189.2 −185.7 arsenic As 33 74.9216 817. (at 28 atmospheres) 613. (sublimates) astatine At 85 (210) 302. (est.) 337. (est.) barium Ba 56 137.33 725. 1640. berkelium Bk 97 (247) 1050. 2590. beryllium Be 4 9.01218 1278. ±5 2970. bismuth Bi 83 208.9804 271.3 1560. ±5 bohrium Bh 107 (262) — — boron B 5 10.81 2300. 2550. (sublimates) bromine Br 35 79.904 −7.2 58.78 cadmium Cd 48 112.41 320.9 765. calcium Ca 20 40.08 839. ±2 1484. californium Cf 98 (251) 900. 1470. carbon C 6 12.011 ∼3550. 4827. cerium Ce 58 140.12 799. 3426. cesium Cs 55 132.9054 28.40 669.3 chlorine Cl 17 35.453 −100.98 −34.6 chromium Cr 24 51.996 1857. ±20 2672. cobalt Co 27 58.9332 1495. 2870. copper Cu 29 63.546 1083.4 ±0.2

Element, in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. A substance such as a compound can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved. An element can, however, be decomposed into simpler substances, such as protons and neutrons or various combinations of them, by the methods of particle physics, e.g., by bombardment of the nucleus. The Atom The smallest unit of a chemical element that has the properties of that element is called an atom . Many elements (e.g., helium) occur as single atoms. Other elements occur as molecules made up of more than one atom. Elements that ordinarily occur as diatomic molecules include hydrogen, nitrogen, oxygen, and the halogens, but oxygen also occurs as a triatomic form called ozone. Phosphorus usually occurs as a tetratomic molecule, and crystalline sulfur occurs as molecules containing eight atoms. Atomic Number a

Mass number, often represented by the symbol A, the total number of nucleons (neutrons and protons) in the nucleus of an atom . All atoms of a chemical element have the same atomic number (number of protons in the nucleus) but may have different mass numbers (from having different numbers of neutrons in the nucleus). Atoms of an element with the same mass number make up an isotope of the element. Different isotopes of the same element cannot have the same mass number, but isotopes of different elements often do have the same mass number, e.g., carbon-14 (6 protons and 8 neutrons) and nitrogen-14 (7 protons and 7 neutrons).

Atomic mass unit or amu, in chemistry and physics, unit defined as exactly 1-12 the mass of an atom of carbon-12, the isotope of carbon with six protons and six neutrons in its nucleus. One amu is equal to approximately  1.66 × 10 −24  grams.

Atomic mass, the mass of a single atom , usually expressed in atomic mass units (amu). Most of the mass of an atom is concentrated in the protons and neutrons contained in the nucleus. Each proton or neutron weighs about 1 amu, and thus the atomic mass is always very close to the mass number (total number of protons and neutrons in the nucleus). Atoms of an isotope of an element all have the same atomic mass. Atomic masses are usually determined by mass spectrography (see mass spectrograph ). They have been determined with great relative accuracy, but their absolute value is less certain.

Atomic weight, mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element , as contrasted with atomic mass , which is the mass of any individual isotope. Although the first atomic weights were calculated at the beginning of the 19th cent., it was not until the discovery of isotopes by F. Soddy (c.1913) that the atomic mass of many individual isotopes was determined, leading eventually to the adoption of the atomic mass unit as the standard unit of atomic weight. Effect of Isotopes in Calculating Atomic Weight Most naturally occurring elements have one principal isotope and only insignificant amounts of other isotopes. Therefore, since the atomic mass of any isotope is very nearly a whole number, most atomic weights are nearly whole numbers, e.g., hydrogen has atomic weight 1.00797 and nitrogen has atomic weight 14.007. However, some elements have more than one principal isotope, and the atomic weight for such an element—since it is a

Isotope, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F. Soddy in explaining aspects of radioactivity; the first stable isotope (of neon) was discovered by J. J. Thomson. The nuclei of isotopes contain identical numbers of protons, equal to the atomic number of the atom, and thus represent the same chemical element, but do not have the same number of neutrons. Thus isotopes of a given element have identical chemical properties but slightly different physical properties and very different half-lives, if they are radioactive (see half-life). For most elements, both stable and radioactive isotopes are known. Radioactive isotopes of many common elements, such as carbon and phosphorus, are used as tracers in medical, biological, and industrial research. Their radioactive nature makes it possible to follow the substances in their paths through a plant or animal body

Mass spectrograph, device used to separate electrically charged particles according to their masses; a form of the instrument known as a mass spectrometer is often used to measure the masses of isotopes of elements. J. J. Thomson and F. W. Aston showed (c.1900) that magnetic and electric fields can be used to deflect streams of charged particles traveling in a vacuum, and that the degree of bending depends on the masses and electric charges of the particles. In the mass spectrograph the particles, in the form of ions, pass through deflecting fields (produced by carefully designed magnetic pole pieces and electrodes) and are detected by photographic plates. The beam of ions first passes through a velocity selector, consisting of a combination of electric and magnetic fields that eliminates all particles except those of a given velocity. The remaining ion beam then enters an evacuated chamber where a magnetic field bends it into a semicircular path ending at the photographic plate. The

Prism, in optics, a piece of translucent glass or crystal used to form a spectrum of light separated according to colors. Its cross section is usually triangular. The light becomes separated because different wavelengths or frequencies are refracted (bent) by different amounts as they enter the prism obliquely and again as they leave it (see refraction ). The shorter wavelengths, toward the blue or violet end of the spectrum, are refracted by the greatest amount; the longer wavelengths, toward the red end, are refracted the least. The Nicol prism is a special type of prism made of calcite; it is used for polarization of light.